P428: SYK INHIBITION DRIVES DEEP RESPONSES IN A BIOMARKER GUIDED SUBSET OF AML ALONE AND IN RATIONAL COMBINATIONS
Authors: M. McKeown, L. A. Carvajal, M. A. Day, C. Noe, P. Kumar, J. DiMartino, D. Saffran
Journal: HemaSphere (2022)

Background: Spleen tyrosine kinase (SYK) acts as a key integrator of signals from cell surface receptors containing an immunoreceptor tyrosine-based activation motif to boost cellular proliferation. In acute myeloid leukemia (AML), SYK serves as a relay to an oncogenic transcriptional regulatory network linked to NPM1, HOXA9 and MEIS1. The selective, orally bioavailable SYK inhibitor entospletinib (ENTO) has demonstrated clinical activity and tolerability in HOXA9/MEIS1-driven AML. ENTO is currently being investigated in a global Phase 3 trial, AGILITY (NCT05020665), in combination with intensive induction/consolidation chemotherapy in patients with treatment-naive NPM1-mutated (NPM1m) AML. Lanraplenib (LANRA) is a next-generation SYK inhibitor with similar potency and selectivity to ENTO but with more favorable pharmacologic properties that is currently being evaluated in combination with gilteritinib in patients with relapsed or refractory (R/R) FLT3-mutated AML (NCT05028751). Aims: To assess the potential of biomarker-guided responses to ENTO and LANRA in mutationally defined subsets of AML and its activity in combination with other targeted agents in translationally relevant models. Methods: Full kinome profiling was run at fixed 1uM concentration followed by dose response IC50 determination for top hits. AML and lymphoma cell lines were tested for antiproliferative effects using CellTiter Glo (CTG) at 5 days. AML patient-derived ex vivo models were tested in microtiter plate viability assays with CTG or in methylcellulose colony viability assays for the NPM1m/FLT3-ITD model used in combination studies. Results: Kinase selectivity profiling found >10-fold higher selectivity of ENTO and LANRA for SYK vs other kinases with improved potency and selectivity compared with the approved first-generation agent, fostamatinib. SYK inhibition with either agent showed robust anti-proliferative activity in a panel of AML and lymphoma cell lines with varying mutational backgrounds. In addition, synergy was observed when ENTO or LANRA was combined with a Menin inhibitor in MLLr, FLT3-ITD cell lines. LANRA and ENTO showed strong anti-proliferative activity in NPM1m AML patient samples validating NPM1m as a patient selection biomarker for ENTO and LANRA. In an analysis of a large public dataset of ex vivo patient samples, the presence of NPM1m and/or FLT3-ITD was significantly predictive of response to ENTO. In an ex vivo model of NPM1m/FLT3-ITD AML, consistent sub-micromolar response to LANRA was observed. Combinations with AML standard-of-care and investigational agents, including azacytidine, showed at least additive effects. Strong synergy with the JAK inhibitor ruxolitinib was consistent mechanistically with a reporter model that demonstrated the ability of LANRA to block activation of STAT5 in response to proleukemic paracrine signaling. Finally, robust synergy across a range of LANRA concentrations was found when paired with venetoclax and gilteritinib. These results prompted further testing with an in vivo, patient-derived xenograft. Summary/Conclusion: SYK is a promising therapeutic target for subsets of AML patients defined by specific mutations. The highly predictive response for SYK inhibition with concurrent NPM1m/FLT3-ITD mutations and synergy with gilteritinib observed in patient derived models is supportive of its clinical evaluation in combination with a FLT3 inhibitor. Our work supports the rationale for an ongoing Phase 1b/2 study of LANRA in combination with gilteritinib in patients with R/R FLT3-mutated AML.